(1) Field of the Invention
The present invention generally relates to a page printer, and more particularly to a page printer which receives bit-mapped data from a host unit as line data in synchronism with feeding of a printing sheet and prints images based on the line data on the printing sheet.
(2) Description of the Related Art
A conventional type of page printer has an LED (Light-Emitting Diode) array as a light source, which type of page printer is an electrophotographic type. As shown in FIG. 1, this type of page printer 104 receives code data generated by a driver 103 in accordance with operations of an application 102 in a host unit 101. The page printer 104 has a controller 106 including a character generator (CG) which converts the code data into image data (bit-mapped data), a bit map memory 105, a buffer 107, a timing controller 108 and an LED head 109. The buffer 107 stores the image data from the bit map memory 105 as line data corresponding to each line of a printed image. The LED head 109 emits light based on the line data from the buffer 107 at timings controlled by the timing controller 108 so that an exposure process is performed. As a result, an image corresponding to the image data spread on the bit map memory 105 is printed on a printing sheet.
The page printer 104 has also a mechanical unit 111 including motors and a driving controller 110 controlling the mechanical unit 111.
In order to provide an inexpensive page printer, a page printer from which the bit map memory is removed has been proposed. This page printer is formed, for example, as shown in FIG. 2. Referring to FIG. 2, in a host unit 121, a driver 123 converts code data into image data (bit-mapped data) based on an application 122. The image data is supplied to a page printer 124. The page printer 124 has a buffer 125, an LED head 127, a timing controller 126, a driving controller 128 and a mechanical unit 129 including motors. The image data from the host unit 121 is stored in the buffer 125 line by line. The LED head 127 emits light based on the image supplied from the buffer 125 at timings controlled by the timing controller 126, so that an exposure process is performed on an image retaining body such as a photosensitive body.
In the above page printer having no bit map memory, a visible image is formed on a printing sheet in accordance with an electrophotographic process. That is, an electrostatic latent image is formed on the image retaining body such as a photosensitive drum, and the electrostatic latent image is developed. The developed image (e.g., a toner image) is then transferred from the image retaining body to a printing sheet. The image formed on the printing sheet is fixed by a fixer. The image data corresponding to the visible image must be supplied to the LED head line by line in synchronism with feeding of the printing sheet without interruption.
However, the host unit must carry out processes other than processes for converting the code data into the bit-mapped data (the image data) and for supplying the bit-mapped data to the page printer. Thus, the conversion of the code data into the image data may be delayed in the host unit. If the supply of the image data for some lines to the page printer is interrupted, some white lines are formed in an image formed on the printing sheet. To prevent this, the host unit must preferentially carry out the process for supplying the image data to the page printer. As a result, in a case where the printing operation of the page printer is performed, the host unit does not have sufficient time for other processes.